Many prior vehicle drivetrain transfer cases incorporate a planetary gear set for providing either two or four wheel driving of an associated vehicle. See, for example, U.S. Pat. No. 4,070,591 which discloses this type of transfer case.
In the four wheel mode of operation where both the front and rear axles are driven through the transfer case, it is also conventional to either interlock the two axles for constant speed driving without any slippage with respect to each other or to provide a differential action between the the two axles in order to permit torque splitting and different speeds between the two axles which is necessary in certain driving conditions to prevent torque buildup in the drivetrain. Usually the differential action is provided by interconnecting front and rear axle outputs of the drivetrain through a differential of the type including beveled side gears and beveled pinion gears meshed with each other. Such beveled gear differentials are conventionally located adjacent the rear wheel output at a location on the opposite side of a front wheel output gear or sprocket from the transfer case input. As disclosed by U.S. Pat. No. 4,299,140, transfer case differentials so located adjacent one of the outputs have also previously utilized a planetary gear set including an outer annular gear, a carrier rotatable concentric with the outer annular gear and having pinions rotatably supported thereon about associated axes parallel with the central axis of the outer annular gear, and a sun gear also rotatable about the same axis as the outer annular gear. This type of planetary gear set provides interaxle differential action with a much more compact configuration than the beveled gear type of differential. However, the positioning of this type of differential adjacent one of the outputs in a conventional manner results in a more space consuming construction than is desirable.
Other transfer cases and vehicle drivetrain power transmission units and the like are disclosed by U.S. Pat. Nos. 3,378,093; 3,407,893; 3,492,890; 3,760.922; 4,215,593; and 4,344,335.